This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 9-295519, filed Oct. 28, 1997, the entire contents of which are incorporated herein by reference.
The present invention relates to a fuel cell, and more particularly to a fuel cell having improved gas manifolds in order to reduce the weight and cost thereof.
A fuel cell has a structure that hydrogen obtained by reforming hydrocarbon fuel, such as natural gas or methane gas, and air which is an oxidizer are supplied to the body of the fuel cell. As a result, electro-chemical reactions are caused to occur through an electrolyte, such as phosphate solution, so that electric energy is generated. A plurality of the single cells each having the power generating function are stacked so that a cell stack structure is formed.
FIG. 1 is an exploded perspective view showing a conventional cell stack structure of a fuel cell. That is, a single cell 1 of a fuel cell body is structured such that a fuel electrode 3, to which hydrogen is supplied in a direction indicated by an arrow A shown in FIG. 1, is disposed on either side of a matrix layer 2 having an electrolyte. Moreover, an air electrode 4 is disposed on the other side of the matrix layer 2, the air electrode 4 being supplied with air in a direction indicated by an arrow B shown in the drawing. Moreover, grooved electrode substrates 5 and 6 are stacked to interpose the fuel electrode 3 and air electrode 4. Moreover, a separator 7 is stacked adjacent to either of the grooved electrode substrate 5 or 6. A cooling plate 8 is stacked whenever a plurality of the single cells 1 are stacked so that one sub-stack 9 is formed. A multiplicity of the sub-stacks 9 are stacked so that a cell stack 10 is formed.
A clamping plate 11 is joined to each of the uppermost portion and the lowermost portion of the cell stack 10. The cell stack 10 and the upper and lower clamping plates 11 are clamed by tie rods 12 so that an integrated cell stack 13 is formed.
As shown in FIG. 2, a pair of fuel-gas manifolds 15a and 15b opposing to each other and a pair of air-gas manifolds 16a and 16b opposing to each other are joined to four side surfaces of the cell stack 13 structured as described above. Thus, fuel gas and air flow perpendicular to each other. A gasket 18 is disposed between the cell stack 13 and each of the gas manifolds 15a and 15b and air-gas manifolds 16a and 16b to prevent rise in a problem, such as deterioration in the power generating efficiently caused from leakage of air or the fuel gas.
The overall body of the conventional gas manifold is made of a metal material. Since the fuel cell is operated at high temperatures of about 200xc2x0 C., great heat radiation takes place from the metal gas manifold. As a result, excessively great energy loss takes place. Therefore, a heat insulating material (not shown) is applied to the outer surface of each of the metal gas manifolds.
When fuel and air are supplied to the gas manifolds 15a and 16b respectively, a portion of the phosphoric acid, with which the matrix layer 2 of the single cell 1 and the grooved electrode substrates 5 and 6 forming the cell stack 10 are impregnated, is diffused in the flows of the fuel gas and air. As a result, the portion of the phosphoric acid is, in the form of steam of phosphoric acid, discharged to the outside of the cell stack (that is, to the inside portion of the gas manifolds).
Since the temperatures of the gas manifolds are somewhat lower than those of the cell stack, a portion of steam of the phosphoric acid discharged to the inside portions of the gas manifolds is, however, condensed and allowed to adhere to the inner walls of the gas manifolds. If the fuel gas and air containing phosphoric acid are brought into direct contact with the inner surfaces of the metal gas manifolds, the metal gas manifolds are vigorously eroded. As a result, holes are undesirably quickly formed.
To overcome the above-mentioned problem, a method of protecting the gas manifolds from phosphoric acid has been disclosed in Jpn. Pat. Appln. 4,950,563, in which the inner surfaces of the gas manifolds are coated with fluororesin.
However, the method of coating the inner surfaces of the gas manifolds with the fluororesin having the following problems cannot completely prevent erosion of the gas manifolds by dint of the phosphoric acid.
That is, the method of coating the inner surfaces of the gas manifolds with the fluororesin suffers from a problem in that phosphoric acid is undesirably introduced through a pin hole. The coefficient of linear expansion of resin coating is about ten times that of the gas manifold. Therefore, the coating method encounters defective adhesion of the resin coating owning to repetition of change in the temperature caused from start and interruption of the operation and change in the load. Therefore, there arises a problem in that the coating is separated.
Since the coating has a relatively small thickness, the phosphoric acid can easily penetrate the coating. Therefore, there arises a problem in that the matrix is eroded and realized reliability is unsatisfactory. To improve the reliability of the coating, the thickness of the coating film must be enlarged. Therefore, heating, coating and cooling processes must be repeated many times. As a result, a long time and great labor are required to complete the above-mentioned processes. Since the coating process and the process for manufacturing the gas manifolds are performed in series, the manufacturing process cannot be shortened.
In addition to the above-mentioned problems, the conventional gas manifold made of a metal material and thus having a great weight must have strong joining and holding structures. Moreover, great clamping force is required. As a result, there arises a problem in that the cost cannot be reduced.
When the gas manifolds are inspected, the heat insulating materials must be separated. Then, the heavy gas manifolds disposed on the four side surfaces of the cell stack cell must be removed. Therefore, the inspection cannot easily be performed and a long time is required.
A first object of the present invention is to provide a fuel cell incorporating gas manifolds each having light weight and small cost.
A second object of the present invention is to provide a fuel cell having gas manifolds each having a simple structure and permitting inspection to easily be performed.
A third object of the present invention is to provide a fuel cell which is capable of preventing leakage of gas from a corner of a cell stack.
A fourth object of the present invention is to provide a fuel cell having gas manifolds with which areas for joining the gas manifolds to the cell stack can be reduced.
To achieve the foregoing objects, according to concept 1 of the present invention, there is provided a gas manifold which is disposed on each of side surfaces of a cell stack, wherein each of the gas manifolds comprises: plate-like heat insulating structures disposed on the side surfaces of the cell stack to oppose each other; and corner members disposed in corners of the cell stack.
According to concept 1 of the invention having the above-mentioned structure is able to significantly reduce the weight as compared with the conventional structure in which the overall body of the gas manifold is made of a metal material. Since the weight can be reduced, assembly and decomposition can easily be performed. Since the channel-shape corner member is disposed at each corner, a stable shape of the gas manifold can be maintained. Moreover, uniform distribution of gas to the cell stack can be performed.
A gas manifold according to concept 2 of the present invention has a structure according to concept 1, wherein the heat insulating structure comprises an internal sheet member and an external heat insulating member which oppose the cell stack.
According to concept 2 of the present invention having the above-mentioned structure, the plate-like heat insulating structure which constitutes the gas manifold is composed of the sealing member and the heat insulating member. Moreover, the sheet member is constituted by a uniform and heat- and phosphoric acid resisting sheet. Therefore, phosphoric acid can reliably be blocked and the cost can be reduced.
A gas manifold according to concept 3 of the present invention has a structure according to concept 2, wherein the sheet member and the heat insulating member has a flange hole through which gas is communicated, and the sheet member has a cylindrical sleeve.
According to concept 3 of the present invention having the above-mentioned structure, the internal sealing member and the cylindrical sleeve are integrated. Therefore, the structure can be simplified. Moreover, phosphoric acid can reliably be blocked. Therefore, leakage of the phosphoric acid to the outside of the system can be prevented.
A gas manifold according to concept 4 of the present invention has a structure according to concept 1, further comprises a first sealing member disposed between the heat insulating structure and the corner member and a second sealing member disposed between the corner member and the cell stack.
According to concept 4 of the present invention having the above-mentioned structure, gas leak between the heat insulating structure and the corner member can reliably be prevented. Moreover, gas leak between the corner member and the cell stack can reliably be prevented.
A gas manifold according to concept 5 of the present invention has a structure according to concept 1, wherein the second sealing member is a frame-like member which is previously hermetically joined to the corner member.
According to concept 5 of the present invention having the above-mentioned structure, gas leak between the corner member and the cell stack can reliably be prevented. Moreover, the sealing member can easily be joined.
A gas manifold according to concept 6 of the present invention has a structure according to concept 1, further comprising upper and lower clamping plates for clamping the cell stack, wherein a planar size of each of the upper and lower clamping plates is larger than a planar size of the cell stack, corner portions of the upper and lower clamping plates have cut portions for receiving the corner members, and the corner members are disposed in the cut portions through cushion members.
According to concept 6 of the present invention having the above-mentioned structure, the gas manifold can easily be assembled. Moreover, sealing can reliably be established while erosion by dint of phosphoric acid is prevented. Since the cushion member having the same shape as that of the cut portion is disposed in the cut portion formed in each of the corner portions of the upper and lower clamping plates, the difference in the thermal expansion between the corner member and the cell body can be absorbed. Moreover, the gas can be sealed.
A gas manifold according to concept 7 of the present invention has a structure according to concept 1, wherein a portion of each of the upper and lower clamping plates projecting over the cell stack is subjected to a phosphoric-acid resisting process.
According to concept 7 of the present invention having the above-mentioned structure, erosion of the cell stack by dint of phosphoric acid and phosphoric acid steam can be prevented.
A gas manifold according to concept 8 of the present invention has a structure according to concept 1, wherein the phosphoric acid resisting process to which the projection portion of each of the upper and lower clamping plates over the cell stack is subjected is fluororesin coating.
According to concept 8 of the present invention having the above-mentioned structure, erosion of the cell stack by dint of phosphoric acid and phosphoric acid steam can reliably be prevented.
A gas manifold according to concept 9 of the present invention has a structure according to concept 1, wherein the phosphoric acid resisting process to which the projection portion of each of the upper and lower clamping plates over the cell stack is subjected is a lining of fluororesin film.
According to concept 9 of the present invention having the above-mentioned structure, erosion of the cell stack by dint of phosphoric acid and phosphoric acid steam can reliably be prevented.
A gas manifold according to concept 10 has a structure according to concept 1, wherein the phosphoric acid resisting process to which the projection portion of each of the upper and lower clamping plates over the cell stack is subjected is a lining of fluorine rubber sheet.
According to concept 10 of the present invention having the above-mentioned structure, erosion of the cell stack by dint of phosphoric acid and phosphoric acid steam can reliably be prevented.
A gas manifold according to concept 11 of the present invention has a structure according to concept 1, wherein a space is formed among the heat insulating structure, the corner member and the cell stack by performing clamping with a clamping member.
According to concept 11 of the present invention having the above-mentioned structure, the heat insulating structure, the corner member and the cell stack can easily and quickly be clamped while a space is being formed among the foregoing elements.
A gas manifold according to concept 12 of the present invention has a structure according to concept 1, wherein the clamping member for clamping the heat insulating structure, the corner member and the cell stack such that a space is formed among the foregoing elements is structured such that ends of angled structures arranged to hold four heat insulating structures corresponding to four side surfaces of the cell stack from rear positions are connected to one another between the heat insulating structures among the four corners by studs through disc springs so as to be clamped.
According to concept 12 of the present invention having the above-mentioned structure, the heat insulating structure, the corner member and the cell stack can easily be clamped while a space is being formed among the foregoing elements.
A gas manifold according to concept 13 of the present invention has a structure according to concept 1, wherein the clamping member for clamping the heat insulating structure, the corner member and the cell stack such that a space is formed among the foregoing elements is structured such that the portions among four heat insulating structures corresponding to the side surfaces of the cell stack are clamped by U-arm toggle clamps in place of the disc springs and studs.
According to concept 13 of the present invention having the above-mentioned structure, the heat insulating structure, the corner member and the cell stack can easily and quickly be clamped while a space is being formed among the foregoing elements.
A gas manifold according to concept 14 of the present invention has a structure according to concept 1, wherein the clamping member for clamping the heat insulating structure, the corner member and the cell stack such that a space is formed among the foregoing elements is structured such that the portion around the four heat insulating structures corresponding to the side surfaces of the cell stack are clamped by a thin stainless steel band and spring buckles.
According to the concept 14 of the present invention having the above-mentioned structure, the heat insulating structure, the corner member and the cell stack can easily and quickly be clamped while a space is being formed among the foregoing elements.
A gas manifold according to concept 15 has a structure according to concept 1, a center clamping structure is provided for each of centers of upper and lower surfaces of the heat insulating structure and centers of upper and lower clamping plates opposing the centers of the upper and lower surfaces.
According to concept 15 of the present invention having the above-mentioned structure, the overall portion of the heat insulating structure including the upper and lower portions can averagely be clamped while a space is being formed among the foregoing elements.
A gas manifold according to concept 16 of the present invention has a structure according to concept 1, wherein supports for locating the vertical positions of the heat insulating structures and jack bolts which are capable of moving and adjusting the heat insulating structures are provided for four corners of the lower clamping plate.
According to concept 16 of the present invention having the above-mentioned structure, locating can easily be performed when the heat insulating structures are joined. Thus, the workability can be improved.
A gas manifold according to concept 17 of the present invention has a structure according to concept 2, wherein the heat insulating member is constituted by inserting a heat insulating material into a heat-insulating-material holding frame.
According concept 17 of the present invention having the above-mentioned structure, the heat insulating material can easily be held. In addition, the overall mechanical strength can be increased. Thus, the heat insulating material can uniformly be clamped onto the side surface of the cell stack. Therefore, the sealing performance can significantly be improved.
A gas manifold according to concept 18 of the present invention has a structure according to concept 17, wherein the sheet member is formed into a plate-like shape which is larger than the heat-insulating-material holding frame.
According to claim 18 of the present invention having the above-mentioned structure, satisfactory insulation can be realized with a simple structure and, thus, the workability can be improved.
A gas manifold according to concept 19 of the present invention has a structure according to concept 17, wherein the sheet member is molded into a box-like shape to fit to the shape of the heat-insulating-material holding frame.
According to concept 19 of the present invention having the above-mentioned structure, the heat and phosphoric acid resisting sheet can previously be molded into the box-like shape to fit to the shape of the heat-insulating-material holding frame. Therefore, the sheet member can be manufactured in a process which is individual from a process for manufacturing the gas manifold. Therefore, time required to manufacture the gas manifold can be shortened.
A gas manifold according to concept 20 of the present invention has a structure according to concept 2, wherein the sealing member is composed of at least one or more types of a PFA (tetrafluoroethylene and perfuloroalkoxyethylene copolymer) resin sheet, a PTFE (tetrafluoroethylene) resin sheet and a FEP (tetrafluoroethylene and propylene hexafluoride copolymer) resin sheet.
According to concept 20 of the present invention having the above-mentioned structure, a sealing member can be formed through which phosphoric acid cannot considerably penetrate and which has excellent heat resistance and mechanical strength.
A gas manifold according to concept 21 of the present invention has a structure according to concept 2, wherein the thickness of the sheet member is not shorter than 0.1 mm nor longer than 1 mm.
According to concept 21 of the present invention having the above-mentioned structure, the amount of penetration of phosphoric acid can be reduced. Thus, a sealing member having excellent mechanical strength and electric insulation can be formed.
A gas manifold according to concept 22 of the present invention has a structure according to concept 2, wherein the sealing member is loosely joined to the heat insulating member.
According to concept 22 of the present invention having the above-mentioned structure, the sheet member comprising the heat and phosphoric acid resisting sheet is not secured to the inner surface of the heat-insulating-material holding frame opposing the cell stack. Since the sheet member is loosely joined to the inner surface, the difference in the thermal expansion between the heat-insulating-material holding frame and the sealing member can be absorbed.
A gas manifold according to concept 23 of the present invention has a structure according to concept 2, wherein the sheet member is composed of at least one or more resin sheets.
According to concept 23 of the present invention having the above-mentioned structure, the amount of penetration of phosphoric acid can reliably by reduced. If a pinhole is formed in one sheet and thus phosphoric acid penetrates the sheet, the residual sheets are able to block the phosphoric acid. Therefore, the reliability can significantly be improved. Since the plural resin sheets are employed, air layers formed among the sheets enable a further excellent heat insulating effect to be obtained.
A gas manifold according to concept 24 of the present invention has a structure according to concept 2, wherein the corner member is made of a metal material or FRP and the inner surface of the corner member is applied with a liner sheet which is any one of a PFA (tetrafluoroethylene and perfuloroalkoxyethylene copolymer) resin sheet, a PTFE (tetrafluoroethylene) resin sheet and a FEP (tetrafluoroethylene and propylene hexafluoride copolymer) resin sheet.
According to concept 24 of the present invention having the above-mentioned structure, the corner members constituting the side portions of the gas manifold are made of the metal material or FRP so that the mechanical strength of the corner member is decreased. Therefore, the shape of the gas manifold can completely be maintained. Therefore, the gas can uniformly be distributed to the cell stack. Since the corner member is applied with the liner sheet, erosion of the corner member by dint of phosphoric acid can be prevented and deterioration in the mechanical strength of the corner member can be prevented.
A corner member according to concept 25 of the present invention has a structure according to concept 24, wherein a corner member which is disposed in the portion of a cooling plate inserted into the cell stack from which an outlet pipe is extended has a structure which is separable horizontally along the corner of the cell stack.
According to concept 25 of the present invention having the above-mentioned structure, the outlet pipe does not obstruct the operation for disposing the corner member to the corner of the cooling plate of the cell stack from which the outlet pipe is extended. Therefore, installation can easily be performed. Moreover, repair can be performed such that the corner member is removed without a necessity of disconnecting the pipe connected to the outlet pipe.
A gas manifold according to concept 26 of the present invention has a structure according to concept 13, wherein a corner member of the corner members which is disposed in the portion of a cooling plate inserted into the cell stack from which an outlet pipe is extended has through holes formed at positions corresponding to the positions of the outlet pipes, holes each of which is smaller than the through holes in the corner member are formed at positions of the corner member corresponding to the liner sheet, and a soft sealing member is disposed around the outlet pipe.
According to concept 26 of the present invention having the above-mentioned structure, the soft sealing members are disposed between the through holes formed at predetermined positions of the corner member and the outlet pipes. Therefore, when the gas manifold is strongly clamped, the soft sealing members overflow the through holes. Thus, the boundaries between the through holes and the outlet pipes can be sealed.
A gas manifold according to concept 27 of the present invention has a structure according to concept 14, wherein the soft sealing member is a disc-like sponge rubber, a hole smaller than the outlet pipe is formed in the central portion of the disc-like sponge rubber, a circumferential slit is formed in the central portion of the disc-like sponge rubber in a direction of the thickness of the disc-like sponge rubber, and the liner sheet is inserted into the slit.
According to concept 27 of the present invention having the above-mentioned structure, the elasticity of the sponge rubber enables the gap between the through hole and the outlet pipe to reliably be sealed.
A gas manifold according to concept 28 of the present invention has a structure according to concept 13, wherein a corner member of the corner members which is disposed in the portion of a cooling plate inserted into the cell stack from which an outlet pipe is extended has through holes formed at positions corresponding to the positions of the outlet pipes, and sleeves for inserting the outlet pipes are formed at positions corresponding to the liner sheet.
According to concept 28 of the present invention having the above-mentioned structure, gas leak from the through hole for the outlet pipe of the corner member can reliably be prevented.
A gas manifold according to concept 29 of the present invention has a structure according to concept 15, wherein a corner member of the corner members which is disposed in the portion of a cooling plate inserted into the cell stack from which an outlet pipe is extended has through holes formed at positions corresponding to the positions of the outlet pipes, also through holes are formed at positions corresponding to the liner sheet and the portions around the through holes are flare-machined for inserting the outlet pipes.
According to concept 29 of the present invention having the above-mentioned structure, gas leak from the through holes for the outlet pipes formed in the corner member can reliably be prevented.
A gas manifold according to concept 30 of the present invention has a structure according to concept 15, wherein the sleeve formed in the liner sheet and arranged to insert the outlet pipe and the outlet pipe are clamped such that the corner members are assembled after which a sealing member is provided around the outlet pipe and a band is used to clamp the outside.
According to concept 30 of the present invention having the above-mentioned structure, gas leak from the through hole of the corner member for the outlet pipe can reliably be prevented.
A gas manifold according to concept 31 of the present invention has a structure according to concept 1, wherein a sealing groove is formed in the corner member adjacent to the cell body, and a projection is formed on the outer surface of the corner member.
According to concept 31 of the present invention having the above-mentioned structure, the portion between the corner member and the cell body can reliably be sealed. Since the outward projection portion is provided, shift of the heat insulating structure can be prevented.
A gas manifold according to concept 32 of the present invention has a structure according to concept 17, wherein the heat insulating material which constitutes the heat insulating member has a thermal conductivity of 0.08 (W/mxc2x7K) at 25xc2x0 C.
According to concept 32 of the present invention having the above-mentioned structure, the heat insulating material having the low thermal conductivity is employed. Therefore, heat generated from the cell can reliably be insulated. Thus, the heat insulating efficiency can be improved.
A gas manifold according to concept 33 of the present invention has a structure according to concept 17, wherein the heat insulating material which constitutes the heat insulating member has a density of 0.2 g/cm3.
According to concept 33 of the present invention having the above-mentioned structure, the heat insulating material having the low density is employed. Therefore, the weight of the gas manifold can reliably be reduced.
A gas manifold according to concept 34 of the present invention has a structure according to concept 17, wherein the heat insulating material which constitutes the heat insulating member has a compressibility of 10% or lower when the compressive load is 200 kg/m2.
According to concept 34 of the present invention having the above-mentioned structure, use of the heat insulating material having the low compressibility enables substantially constant thermal conductivity to be realized because the compressibility is not changed considerably even if the pressure of the fuel gas or the oxidizer gas is somewhat changed. Therefore, a heat insulating effect can reliably be obtained.
A gas manifold according to concept 35 of the present invention has a structure according to concept 17, wherein the heat insulating material which constitutes the heat insulating member is composed of a plurality of heat insulating material layers made of at least one type of materials and having different strengths.
According to concept 35 of the present invention having the above-mentioned structure, the heat insulating material is composed of the plural heat insulating material layers. Thus, the characteristics of the heat insulating materials can be used. Therefore, a heat insulating material having a good balance and exhibiting excellent heat insulating performance and satisfactory mechanical strength can be obtained.
A gas manifold according to concept 36 of the present invention has a structure according to concept 35, wherein the heat insulating material which constitutes the heat insulating member is arranged such that a heat insulating material having excellent heat insulation is disposed on the inside (a portion adjacent to the cell body) and a heat insulating material having increased mechanical strength is disposed on the outside.
According to concept 36 of the present invention having the above-mentioned structure, a heat insulating member having excellent heat insulation and mechanical strength can be formed.
A gas manifold according to concept 37 of the present invention has a structure according to concept 35, wherein the heat insulating material which constitutes the heat insulating member is arranged such that a low density heat insulating material composed of fibrous substances having excellent heat insulation is disposed on the inside (portion adjacent to the cell body) and a low-density heat insulating material having a structure containing bubbles, which have excellent mechanical strength, is disposed on the outside.
According to concept 37 of the present invention having the above-mentioned structure, excellent heat insulation and mechanical strength can be obtained. In addition, the weight can be reduced.
A gas manifold according to concept 38 of the present invention has a structure according to concept 17, wherein the thickness of the heat insulating material which constitutes the heat insulating member is 10 mm or smaller.
According to concept 38 of the present invention having the above-mentioned structure, the thickness of the heat insulating material can be reduced. Thus, the size can reliably be reduced as compared with the conventional gas manifold.
A gas manifold according to concept 38 of the present invention has a structure according to concept 17, wherein the heat insulating material which constitutes the heat insulating member has a honeycomb structure, and the honeycomb is filled with the heat insulating material.
According to concept 39 of the present invention having the above-mentioned structure, the strength of the heat insulating material can reliably be increased. Therefore, a heat insulating material having excellent reliability for a long time can be provided.
A gas manifold according to concept 40 of the present invention has a structure according to concept 17, wherein two sides of the heat insulating material constituting the heat insulating member are reinforced by first reinforcing members, and surfaces of the first reinforcing members are reinforced by second reinforcing members.
According to concept 40 of the present invention having the above-mentioned structure, the first reinforcing member, such as a plain lath, a hexagonal wire mesh or punching metal, enables the mechanical strength to be increased. Since the second reinforcing member, such as glass cloth, is used to cover the first reinforcing member, a crack of a sheet member formed by dint of an edge surface of the reinforcing member can be prevented. Thus, breakage caused from concentration of local stresses can be prevented. Thus, a heat insulating member exhibiting reliability for a long time can be provided.
A gas manifold according to concept 41 of the present invention has a structure according to concept 17, wherein the heat-insulating-material holding frame comprises a holding member for holding the clamping member.
According to concept 41 of the present invention having the above-mentioned structure, the holding member provided for the heat-insulating-material holding frame is able to prevent shift of the clamping member. As a result, stable clamping force can be maintained for a long time.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.